Separators play a crucial role in alkaline batteries. They keep the positive and negative sides of the battery separate while letting certain ions go through and blocking others. The separator is a passive element that has to perform the same task unchanged for the life of the battery. Meanwhile, it must be able to withstand a strongly alkaline environment both at ambient and elevated temperatures. In addition, it must be capable of resisting oxidative attacks.
In an alkaline battery, a separator should conduct hydroxyl ions at a sufficiently rapid rate to meet the increasingly high current demands of modern electronics. Films of cellulose in the form of regenerated cellulose have been used since World War II as the separator of choice for this purpose because of its superior ability to conduct hydroxyl ions in strongly alkaline media. Its low electrical resistance of 10 milliohm-in2 has also led to its favor for use in zinc-based batteries, such as silver-zinc, zinc-nickel, and zinc manganese dioxide batteries. Additionally, it acts as a physical barrier to migration of other ions in the battery, such as that of zincate ions and silver ions in a silver-zinc battery.
Despite its advantages as a battery separator, regenerated cellulose has some intrinsic limitations. During overcharge, an alkaline battery tends to break down water and evolve hydrogen in sufficient quantities as to materially affect the internal impedance of the battery. Unless this hydrogen is removed efficiently, a parasitic feedback results in which the battery continues to be overcharged with resultant pressure buildup and venting of hydrogen or catastrophic rupture of the battery case. Regenerated cellulose, however, exhibits one of the lowest hydrogen permeability coefficients of known polymers, reported in the Polymer Handbook as 2.044×10−15 cm3cm−1s−1Pa−1.